1. Field of the Invention
This invention relates to lock-up control devices that control engaging forces of lock-up clutches, which share engine outputs with torque converters to transmit them toward input shafts of automatic transmissions of cars. This application is based on Patent Application No. Hei 9-252533 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
The paper of Japanese Patent Application, Publication No. Hei 7-332479 discloses an example of the lock-up control device that controls engaging force of the lock-up clutch, which shares engine output with the torque converter to transmit it toward the input shaft of the automatic transmission of the car, wherein the engine output is controlled in response to throttle opening. This type of the lock-up control device has an input shaft torque estimation unit, which estimates input shaft torque of the transmission, in other words, turbine torque of the torque converter. So, the device controls the engaging force of the lock-up clutch based on the input shaft torque estimated by the input shaft torque estimation unit.
The above lock-up control device has an advantage in that the aforementioned input shaft torque can be calculated with a high precision. However, in the case where real engine torque sharply and suddenly increases, there is a disadvantage in that the rise timing of the input shaft torque should be delayed from the rise timing of the real engine torque. For this reason, the aforementioned lock-up control device that controls the engaging force of the lock-up clutch on the basis of the input shaft torque suffers from a problem that a delay occurs in control of the lock-up clutch in the case where the real engine torque sharply and suddenly increases. In consideration of the dynamics of the lock-up clutch itself, such a control delay will be increased further more.
Further, there occurs a so-called "chip-in phenomenon", according to which the engine speed decreases because the engaging force of the lock-up clutch increases after the engine speed increases unnecessarily high. In other words, in the chip-in phenomenon, the engine speed increases unnecessarily high, then, the engine speed decreases. This chip-in phenomenon is disadvantageous for the car (and driver) in aspects of drivability and fuel efficiency.